This invention relates generally to techniques for scrambling and unscrambling a video signal and, more particularly, to techniques for reducing distortions in a video signal caused by scrambling the video signal.
Secure transmission of video signals is becoming increasingly important with the growing popularity of video teleconferencing, cable TV and satellite TV transmissions, and with the advent of direct-broadcast-satellite (DBS) transmission. Various techniques have been developed that provide varying levels of video security, with corresponding levels of complexity and cost. One technique that provides a relatively secure video signal with a modest amount of complexity and cost is line spin, or line rotation, scrambling. A typical line spin scrambling system is disclosed in U.S. Pat. No. 4,070,693 to Shutterly.
Line spin scrambling is performed in an encoder by segmenting the active portion of a video line at a breakpoint determined by a pseudorandom number generator. The two segments are then interchanged, or "rotated," while the horizontal and vertical synchronization and blanking intervals are left intact. After transmission and reception of the video signal, the signal is unscrambled in a decoder by reversing the line spin scrambling that was performed in the encoder. The breakpoint is determined in the decoder by an identical pseudorandom number generator that is synchronized with the pseudorandom number generator in the encoder.
Although line spin scrambling offers many advantages, it has certain disadvantages, one of which is an amplitude gap or discontinuity that appears at the point in the scrambled video line where the two segments are pieced together. The amplitude gap results from an amplitude differential between the beginning and the end of the active portion of the video line, prior to line spin scrambling. In other words, the amplitude gap results from a difference in amplitudes measured at the beginning and the end of a video line prior to scrambling, since, after scrambling, these two amplitudes are positioned at the breakpoint. Because the amplitude gap provides information about the location of the breakpoint of the scrambled video line, an unauthorized viewer could unscramble the scrambled video signal by detecting the amplitude gap in each scrambled video line. Furthermore, transmission and reception of the line-spin scrambled video signal causes a distortion in the scrambled video signal in the vicinity of the discontinuity, because of the finite bandwidth of conventional television transmission and reception systems and the very high frequency content of a discontinuity. Accordingly, there has been a long existing need in the broadcasting industry for a method and apparatus for minimizing the amplitude gaps or discontinuities in a line-spin scrambled video signal. The present invention clearly fulfills this need.